Volcanic gases are gases that have escaped or been released from the Earth during volcanic activity. In most cases, these gases were trapped in magma, the superheated, molten rock beneath the Earth’s crust. If magma is ejected during a volcanic eruption, these gases will be violently released into the atmosphere. Some volcanic gases also escape through cracks in the Earth’s surface. Many of these gases can be dangerous or deadly to humans and other living creatures.

The Earth’s surface is a dense layer of solid rock, called the crust, which covers the entire planet. Beneath this layer is the magma, which is constantly moving and changing due to heat and chemical reactions. A volcano is a place where the magma erupts onto the Earth’s surface; this can be caused by seismic activity, or sometimes by the buildup of gases within the magma itself. On the surface, the magma becomes a lava flow, and will harden into rock as it cools. Meanwhile, volcanic gases released with the magma will often react with the surrounding water or atmosphere.

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The most common gas released by volcanoes is simple water vapor, or steam. Other common volcanic gases include carbon dioxide and sulfur dioxide, along with various hydrogen compounds. Some airborne materials released by volcanoes are not gases at all, but great masses of tiny particles, called aerosols. Most gases will disperse into the atmosphere shortly after being released from the volcano, sometimes causing lingering detrimental side effects such as acid rain. Airborne particles such as ash, however, can travel great distances and have been known to affect weather and environmental patterns on a global scale.

Volcanoes pose numerous hazards to living things, and their gases can be deadly. Hydrogen sulfide, hydrogen chloride, and hydrogen fluoride can cause irritation and damage to lung tissues if inhaled. Carbon dioxide is an invisible gas that can collect in low-lying areas and cause unconsciousness and death. These volcanic gases can be released from cracks in the Earth that seem harmless, which is why an experienced guide is essential in areas of high volcanic activity. The presence of certain gases near the surface can also indicate that a volcano is near eruption.

Lava flowing into ocean water creates an acidic, steam-like cloud called a laze, short for “lava haze.” Sulfur dioxide can combine with other chemicals and gases to create volcanic smog called “vog;” both can be harmful to any humans or animals in the immediate vicinity. Sulfur dioxide and carbon dioxide can also create long-lasting environmental effects if they are released into the atmosphere in great quantities. Scientific studies have demonstrated, however, that the atmospheric changes caused by human activity have a greater impact on the environment than those brought on by volcanic gases.

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